Adsorption and Ion Exchange with Synthetic Zeolites - American

10 Lead and Cadmium Ion Exchange of Zeolite NaA ELLIOT P. HERTZENBERG and HOWARD S. SHERRY

Downloaded by GEORGETOWN UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: August 15, 1980 | doi: 10.1021/bk-1980-0135.ch010

The PQ Corporation, Research and Development Division, P.O. Box 258, Lafayette Hill, PA 19444

In recent years the necessity of controlling the heavy metal content of waste water has become more apparent. This need has prompted us to study Pb and Cd^ ion exchange in the synthetic zeolite Na-A. There has been no work published on Pb ion exchange in this zeolite and we are only aware of the work of Gal and coworkers on C d ion exchange of Na-A (1). Gal and Radovanov (2) and Maes and Cremers 0,4) have also reported work on Cd2+ ion exchange of zeolites Na-X and Na-Y. A large body of information has been published on polyvalent ion exchange in zeolites. Examples of alkaline earth ion exchange in zeolites include studies on ion exchange of Na-A (5), Na-X (6,7), Na-Y a,8), zeolite Τ (9), and erionite (10). Examples of Ni +, Co2+ and Zn^+ ion exchange for Na in zeolites include exchange of Na-A ( 1), Na-X (2,3). The work cited above provides a sampling of the variety of ion exchange behavior exhibited in these systems. 2+

+

2+

2+

2

+

Also of relevance i s t r i v a l e n t i o n exchange t y p i f i e d by the work done on L a i o n exchange of Na-X and Na-Y (11). I t i s the purpose o f the work reported h e r e i n t o compare the i o n exchange o f P b and C d f o r N a i n z e o l i t e A t o that o f other ions i n the same and other z e o l i t e s . To f u r t h e r t h i s end, we have s t u d i e d these i o n exchange r e a c t i o n s as a f u n c t i o n o f temperature and d i f f e r e n t anions. 3 +

2 +

2 +

+

Experimental S e c t i o n A l l metal s a l t s were reagent grade. Doubly deionized water was used to prepare the s o l u t i o n s of 0.1N metal i o n . The l e a d acetate s o l u t i o n s were f i l t e r e d through a M i l l i p o r e f i l t e r i n order t o remove a s l i g h t t u r b i d i t y . Z e o l i t e Na-A was synthesized i n our l a b o r a t o r y . A f t e r c r y s t a l l i z a t i o n from the sodium a l u m i n o s i l i c a t e g e l , the z e o l i t e was c a r e f u l l y washed with deionized water i n order to remove occluded i m p u r i t i e s without causing H3O"*" ion exchange. I t was then stored i n a d e s i c c a t o r c o n t a i n i n g saturated NH4CI s o l u t i o n .

0-8412-0582-5/80/47-135-187$05.00/0 © 1980 American Chemical Society

In Adsorption and Ion Exchange with Synthetic Zeolites; Flank, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

188

SYNTHETIC ZEOLITES

The zeolite was highly crystalline and contained negligible amounts of crystalline impurities or amorphous material. Table 1 l i s t s the chemical analysis of the Na-A used i n this work. Table 1


Chemical Analysis of Zeolites Na-A % Wt.

Moles/Mole A l 0

Na 0

17.2

0.98

sio

32.9

1.94

28.9

1

2

2

A1 0 2

3

H0 2

Total

o

Q

21.8 100.8

The atom ratio of Na/Al i n the Na-A was 0.98 and the Si/Al ratio was 0.97. The calculated exchange capacity i s 7.0 milliequivalent per gram (meq/g) of anhydrous zeolite. Phase equilibrium was accomplished by weighing suitable quantities of the sodium form of the zeolite into 125 ml polycarbonate bottles which contained 50 ml of the salt solution. The total salt concentration was 0.100 (±0.010) N, known to three significant figures. At the high end of the isotherm, the starting solution contained only the ingoing cation; at the low end, the solution contained both of the exchanging cations. The equilibrations were carried out for a minimum of three days i n a New Brunswick Scientific Company AQUATHERM Water Bath Shaker at 5°, 25°, and 50°C, with temperature control to +0.5°C. Prior to analysis of the equilibrium solutions, the solid and solution phases were rapidly separated by f i l t r a t i o n through a Millipore f i l t e r immediately after removal from the constant temperature bath. Lead and sodium analyses of the f i l t r a t e were obtained by atomic absorption spectroscopy. The cadmium analyses of the f i l t r a t e were obtained by plasma emission spectroscopy. These analyses showed that two Na ions entered the solution for every C d or P b that l e f t (±2%). +

2 +

2 +

Results and Discussion Lead ion-exchange isotherms are plotted i n Figures 1 and 2. The data i n Figure 1 for the Pb(N03)2~Na-A system show that Na-A is extremely selective for P b and that this selectivity increases with temperature over the range of 50°C. Comparison of this system with the Pb(CH3C00)2-Na-A system (Figure 2) shows 2 +



HERTZENBERG AND SHERRY

189

Pb & Cd Ion Exchange

Figure 1. Pb-Na-A system at 0.1 nor mality: (O) NOf at 5°C, Ο NOf at 25°C «2>)NOfat50°C >

Figure 2. Pb(CH COO) -Na-A system at 0.1 total normality: (Θ) 5°C, Ο 25°C,«T)) 50°C 3

t


SYNTHETIC ZEOLITES

190

that the selectivity of Na-A for Pb i s greater from nitrate solution than from acetate solution and that the temperature effect i s similar. Cadmium ion-exchange isotherms are plotted i n Figures 3 and 4. Zeolite Na-A i s selective for C d over Na ion but P b ion is preferred to an even greater degree. Just as i n the case of P b exchange, Na-A removes more C d from a nitrate solution than from the corresponding acetate solution. The temperature dependence of Cd -Na ion exchange i s negligible within the precision of our experimental data. We have used the phase distribution data shown i n Figures 1-4 to calculate the standard free energies of the ion exchange reaction using a simplified version of the equation of Gaines and Thomas (12). 2 +

2 +

2 +

2 +

2+


+

+

ι

+ l o

*

K

" " 275Ô3

l0

X 8
)50 C s

J )2

I 0.4

ι 0.6

ι 0.8

I ίο

z

s

o


192

SYNTHETIC ZEOLITES

The c o r r e c t e d s e l e c t i v i t y c o e f f i c i e n t i s r e l a t e d t o the e q u i l i b r i u m constant and the s e l e c t i v i t y c o e f f i c i e n t by:

(4)

Κ = Κ

Κ

2 . Vja

= Κ

(5)


c

fa

c

c

3

Γ

δ ΜΧ + 2

+

2+

In equation 3 the terms o f f N a and /"M are the r a t i o n a l a c t i v i t y c o e f f i c i e n t s of exchanging c a t i o n s i n the z e o l i t e phase and the terms i^Na a n d ^ M are the m o l a l s i n g l e i o n a c t i v i t y c o e f f i c i e n t s i n the s o l u t i o n phase. Equation 4 can be r e w r i t t e n as equation 5 when the two s a l t s , NaX and MX2 have a common anion. The mean m o l a l a c t i v i t y c o e f f i c i e n t s u s u a l l y can be estimated from l i t e r a t u r e data. The c o r r e c t e d s e l e c t i v i t y c o e f f i c i e n t i n c l u d e s a term that c o r r e c t s f o r the n o n - i d e a l i t y of the s o l u t i o n phase. Thus any v a r i a t i o n i n the c o r r e c t e d s e l e c t i v i t y c o e f f i c i e n t i s due t o n o n - i d e a l i t y i n the z e o l i t e phase (see equation 3 ) . We have p l o t t e d i n F i g u r e s 5-8 the r a t i o n a l s e l e c t i v i t y c o e f f i c i e n t , % ς , v s . heavy metal i o n loading, Y. The r a t i o n a l s e l e c t i v i t y c o e f f i c i e n t f o r d i - u n i v a l e n t i o n exchange i s defined as +

#

2 +

β

3

Y(l-X) «R.C

2 m

2

(1-Y) X

K

' . _1_ .^ 2 + C

2 N

' T

(7-a)

4

"

NaXi+ 4 K

I c - KR.C.

·

2 N

T

r

(7-b)

.3

If

NaXi MX + 2




193


60

- É1 ο 3.0

ce.

H

ô

8 \ i o

-

o

-

•\

0

1

1

1

0.2

0.4

0.6

Figure 6. Rational selectivity coefficient vs. Pb * ion exchange from CH COO~ solution: (Θ) 5°C, O 25°C (O) 50°C 2

0.8

1.0

s

>


SYNTHETIC ZEOLITES


194

Figure 7. Rational selectivity coefficient vs. Cd ion exchange from NOf solution: (O; 5°C, Ο 25°C (O) 50°C 2+

f

Figure 8. Rational selectivity coefficient vs. Cd * ion exchange from CH COO~ solution: (Θ) 5°C, Ο 25°C, (O) 50°C 2

3


10.


195


Substituting equation 7-b into equation 1 gives

ι

ν

l o g K

=

1 - 2^03

_,_ , +

1

0

8

„„ 0 NaXÎ . ( ΤT - * *R.C. v

dY

(8)

+

MX +


2

where the solution phase activity coefficient term i s assumed independent of Y. The plots i n Figures 5-8 have been used to evaluate the integral i n equation 8 where a reasonable estimate of the activity coefficient term for the solution phase could be made. Standard free energies have been calculated from the equations Δ-ο = - 2.303RT logK kilojoules/equiv Τ G

These data are shown in Table 2. Table 2 Standard Free Energies of Reaction Reaction

Cd

2 +

+ 2Na-A

AG^, Kj/equiv 278°K

298°K

323°K

-10.4

-6.86

-6.40

Enthalpies of reaction have not been calculated because i n most cases our data are not sufficiently accurate to differentiate between 5° and 25°C. However, the data clearly show that Na-A becomes more selective for P b and C d with increasing temperature. Our isotherms also show that Na-A removes more Pb and Cd from nitrate solutions than from acetate solutions. If the data of Gal and coworkers (1) for the CdCl2 + Na-A reaction i s also used, the selectivity of Na-A for Cd as a function of coion exhibits the series 2 +

2 +

2

N0

3

> CH C00 3

+

> Cl


2

+

SYNTHETIC ZEOLITES

196

T h i s sequence f o l l o w s the i n v e r s e o f the sequence o f the degree of complexation o f C d by the coions (13) as shown i n Table 3· 2 +

Table 3 Complexation o f Lead and Cadmium Formation Constant


Metal Pb

Anion

log$i

log$2

OH" CH C00"

7·82 2.52

10.85 4.00

Cl" NO ~

1.62 1.18

2.44

OH" Cl" CH C00"

4.17 1.95 1.5

8.33 2.50 2.3

3

Cd

3

N0

* M

2 +

** M

2 +

+ A" = MA

0.40

3

+

3

+ 2A" = MA

3

2

X

2

We have obtained part of a CdCl -Na-A ion-exchange isotherm i n 0.1N s o l u t i o n a t 25°C to check the r e s u l t s obtained by G a l and coworkers ( 1 ) . These data, shown i n F i g u r e 3 as c r o s s e s , i n d i c a t e that the preference of Na-A f o r C d ions v a r i e s with coion a c c o r d i n g to the s e r i e s 9

2 +

N0 " > CH C00" = C l " . 3

3

We cannot e x p l a i n why we f i n d that Na-A e x h i b i t s a higher selectivity forC d i n the C l " system than G a l Cl) found. Perhaps t h i s d i f f e r e n c e r e l a t e s to the d i f f e r e n c e i n our batches of z e o l i t e . The standard f r e e energies measured f o r the r e a c t i o n 2 +

2 +

+

2 +

+

M ( s o l ) + 2Na (zeol) = M ( z e o l ) + 2Na (sol)



10.


Pb

& Cd

Ion

197

Exchange

should be the same no matter which heavy metal salt i s used, because in the Gaines & Thomas approach (12) corrected selectivity coefficients are used. We are not able to demonstrate that we obtain the same standard free energies i n the nitrate and acetate system because no data are available on the mean molal activity coefficients of Cd(CH3C00)2 and Pb(CH3C00)2. We expected that Gal and coworkers (1) would obtain the same value for the standard free energy for C d ^ exchange of Na-A at 298°K as we did. Our value i s -6.86KJ/equiv and theirs i s 4.56KJ/equiv. We cannot rationalize this difference. However, i t i s clear that our values for the standard free energies of exchange and those of Gal (1) are quite uncertain because of complexation. It i s not l i k e l y that acetato or nitrato complexes of C d and P b w i l l diffuse into zeolite A, because CH3COO" and NO3" are too large. Barrer and Meier (14) have shown that zeolite A does not occlude salt molecules during aqueous ion exchange. They had to use a fused salt system to enable AgN03 to diffuse into Na-A. Furthermore, even on a selectivity basis i t i s l i k e l y that simple divalent cations are strongly preferred to the complex monovalent cations. Thus the exchange reaction can be thought of as a competition for C d and P b ions between the fixed negative charges in the zeolite phase and the mobile, negatively charged ligands i n the solution phase. It i s worth noting that, although we show points on our ion exchange isotherms indicating greater than 100% P b and C d ion exchange, we believe that within our experimental error two Na ions were replaced with one C d or P b ion. We are aware that McCusker and Seff (15) have reported considerable "over-ion exchange." However, they used crystals of Na-A prepared by Charnell's method (16) and did not chemically analyze them. Bas1er and Malwald (17) have recently shown that Na-A crystals prepared by Charnell's method have considerable amounts of aluminum occluded i n the supercages of the zeolite as well as in the sodalite cages, probably as sodium aluminate (they found Si/Al atom ratios as low as 0.88). This excess aluminum could give rise to considerable excess ion-exchange capacity. Basler and Malwald also showed that Na-A, synthesized by the standard method, contained only small amounts of excess aluminum i n the sodalite cages and that this material had an Si/Al ratio of 0.98-0.97. Thus we feel that only a small amount of over-exchange could occur in our case. 2 +

+

2 +

2 +

2 +

2 +

2 +

2 +

2 +


SYNTHETIC ZEOLITES

198

Abstract Lead and cadmium ion exchange of zeolite Na-A has been studied as a function of coion and temperature. It is found that the zeolite is very selective for these ions and that this selectivity is greater in nitrate solutions than in acetate solutions. The temperature dependence of the ion exchange reaction is small but distinguishable in the case of Pb and too small to observe in the case of Cd . 2+

2+


Literature Cited 1. Gal, I. J., Jankovic, O., Malcic, S., Radovanov, P., and Fodorovic, Μ., J . Chem. Soc., Far. Trans., 1971, 67, 999. 2. Gal, I. J., and Radovanov, P., J . Chem. Soc., Far. Trans., 1975, 71, 1671. 3. Maes, Α., and Cremers, Α., J . Chem. Soc., Far. I, 1975, 71, 265. 4. Rosolovskaza, Ε. Ν., Topchieva, Κ. V . , and Dorozkho, S. P., Zhur. Fiz. Khimii, 1977, 51, 1469. 5. Sherry, H. S., and Walton, H. F., J . Phys. Chem., 1967, 71, 1457. 6. Barrer, R. M., Rees, L. V. C., and Shamsuzzoho, J. Inorg. Nucl. Chem., 1966, 28, 629. 7. Sherry, H. S., J . Phys. Chem., 1968, 77, 4086. 8. Barrer, R. Μ., Davies, J . Α., and Rees, L. V. C . , J. Inorg. Nucl. Chem., 1968, 30, 3333. 9. Sherry, H. S., "Proc. Intern. Conf. Ion Exchange in the Process Industries," London, July 1969, 1970, 329. 10. Sherry, H. S., Clays and Clay Minerals, 1979, 27, 231. 11. Sherry, H. S., J . Col. and Inter. Sci., 1968, 28, 288. 12. Gaines, G. L. Jr., and Thomas, H. C . , J . Chem. Phys., 1953, 21, 714. 13. "Langes' Handbook of Chemistry," twelfth ed., J . A. Dean, Ed., McGraw-Hill, Co., N.Y., 1972. 14. Barrer, R. Μ., and Meier, W. Μ., J . Chem. Soc., 1958, 299. 15. McCusker, L. B., and Seff, Κ., J . Am. Chem. Soc., 1978, 100, 5052. 16. Charnell, J . F . , J . Cryst. Growth, 1971, 8, 291. 17. Basler, W. D., and Malwald, W., J . Phys. Chem., 1979, 83, 2148. RECEIVED April 24, 1980.


Adsorption and Ion Exchange with Synthetic Zeolites - American

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